Twice in the past, the yellowtail Seriola quinqueradiata TEMMINCK et SCHLEGEL was cultured on a large scale in the coastal waters of the Amami and Ryukyu Islands which are remote from its natural habitat, that is, the coastal waters of the Japanese main lands of Honshu, Shikoku and Kyushu. The seedlings used were juveniles reared in farms in Shikoku and Kyushu and the foods used were the same frozen trash fishes as were commonly used in farms in the main lands.Unexpectedly, in both cases most of the cultured yellowtails were seriously affected by kudoasis. All affected fish lost entirely their commercial value, because numerous white spherical or elliptical trophozoites, mostly 1-5 mm in size, of Kudoa sp, were formed in the skeletal muscle of all parts of the body.Field studies made by the present authors for the past 3 years revealed the fact that coral fishes in Amami and Ryukyu waters, that is, Abudefduf sexfasciatus, A. vagiensis, Chromis isharai, C. notatus and Chrysiptera assimilis (Percida : Pomacentridae), occasionally carried trophozoites of the same Kudoa sp. in their skeletal muscle, though in these fishes the number of trophozoites in an individual fish was small, one in almost all cases. A scanning-electron-microscopic examination demonstrated that the spores of Kudoa collected from yellowtails were identical in morphology with those from coral fishes. A new name Kudoa amamiensis was tentatively given to this Kudoa.Through various field studies and experiments a hypothesis is possible that the source of infection of kudoasis in yellowtails cultured in Ryukyu and Amami waters may be the above-mentioned coral fishes and yellowtails may become seriously infected by eating coral fishes bearing sporulated trophozoites of the myxosporidia which happen to swim into net pens for yellowtails.

For the control of fish microsporidiosis, life history of the organism, process of infection and the possibility of chemotherapy, immunity and other remedy are discussed in this paper.Water temperature heavily influenced upon the shizoglony and sporogony of G. takedai, G. stephani, and G. plecoglossi.Artificial injection methods were established either by the skin or the oral routes.Drying, freezing, treatment with hot water, ultraviolet ray, cation surface active agents and chloric drugs were found effective for the inactivation of the spores of G. plecoglossi.Fumagillin was found to be promissing for the control of microsporidiosis (G. plecoglossi and P. anguillarum). Its effective dosage differed with species.The rainbow trout once infected with G. takedai seems to aquire strong immunity to the disease.

A microsporidiosis of salmonid had previously been found in limited districts in Hokkaido, i. e. River Chitose and Lake Tokito. In September of 1977, a new epizootic area of this disease was confirmed by the author at Lake Akan in the eastern Hokkaido. In this report, the similarity of biological environment among epizootic areas especially the habitability for fresh pearl mussel, Margaritifra laevis is discussed.

Two hundred fifty nine strains of Vibrio anguillarum were isolated from cultured ayu of various districts in Japan from 1974 to 1977. These strains were tested for their susceptibility to 6 chemotherapeutic agents : sulfamonomethoxine, chloramphenicol, tetracycline, nalidixic acid, nitrofurazone and trimethoprim. Nine of the 259 strains were sensitive to all drugs tested. The remaing 250 strains were resistant to various combinations of the six drugs tested. Transferable R plasmids were detected in 165 out of the 250 resistant strains, and strains from all districts turned out to have some transferable R plasmids. This high incidence of drug resistant strains of V. anguillarum was assumed to be due to the selective pressure exerted by chemotherapeutic agents used during culturing the fish. These strains fall into at least three different O sero-groups.The efficacy of hyperosmotic infiltration technique (HI) in controlling vibriosis of ayu was tested. Experimental fish were first placed in a solution containing 5.32% NaCl for 2 min. and then placed in a vaccine solution of V. anguillarum (5.0 × 108/ml) bacterin for 3 min. A month after immunization, the immunized fish were found to be resistant to V. anguillarum to intraperitoneally injected.In the second experiment, formalin-added culture of V. anguillarum was centrifuged and the precipitated bacterial cells were resuspended in saline solution of the same volume as the original culture. As in the first experiment, the fish were held in a solution of 5.32% NaCl for 2 min, and then placed either in the above suspension or the medium supernatetant for 3 min. Immersion of fish in the suspension provided adequate protection but immersion of fish in the supernatant was not effective to protect against intraperitoneally injected V. anguillarum a month after vaccination. Treatment with the supernatant caused high mortality to ayu presumably due to the exotoxin from bacteria but immersion in the Vibrio cell suspension did not cause higher mortality than a control group immersed only in 5.32% NaCl solution during are one month.In the third experiment, the fish were immersed in 5.32% NaCl solution containing lyophilized whole cells of V. anguillarum for 3 min. The vaccinated fish turned out to be well protected against V. anguillarum intraperitoneally injected a month after treatment. The HI method could be used most effectively to immunize ayu against vibriosis.

Since 1967 the damage of cultured yellowtails by nocardial infection has been increasing year after year, and it is urgently required to take some effective countermeasures. However, chemotherapy for this infection has not yet been established, partly because tubercles formed in the diseased fish rendered drugs loss effective. Preventive measures by the use of biological characteristics of the organism and by vaccination are briefly discussed in this paper.1. Preventive and Control Steps from Aspects of Viability of Pathogenic Organisms in Seawater and Mode of Infection:In the seawater, the pathogenic bacteria could not survived long, especially all organisms being killed in the open seawater within 2 days. However, they survived more than 90 days by the addition of fish extracts at a level of 100 ppm to the open seawater. In the survery of bacterial distribution at culture farms by the fluorescent antibody method, a large number of fluorescent-positive bacteria were detected during the summer when the seawater was heavily polluted. Meanwhile, the challenge test with the water suspended with virulent bacteria revealed that the gill and the digestive system were most vulnerable to the bacteria, and that the higher the bacterial level in the bath, the greater the bacterial count in the challenged fish. From these results it can be assumed that, although the pathogenic bacteria cannot survive long in fresh open seawater, their viability is greatly strengthened in the polluted environment. Nutrients from the feed may participate in this pollution. These conditions would provide a favorable habitat for pathogenic bacteria, and thus, favor the infection if their susceptibility increases. The primary preventive steps will therefore be to minimize the outflow of extracts from the feed during thawing in the seawater, and to avoid overcrowding and overfeeding, thereby keeping the culturing enviromnent clean. Further, as injuries and malnutition of fishes are the greatest factors to increase their susceptibility, it will be essential to avoid injuries and to keep the fishes in nutritionally good condition. As sanitary procedures, accidental introduction of carrier and diseased fishes should be avoided, and dead fish must be incinerated for the prevention of further spread of the disease.2. Prevention with Vaccination:When cultured yellow tails were inoculated with killed bacteria, their serum antibody titers rose to 10, 000 in 6 weeks. Further, the effectiveness of vaccination was also demonstrated by the results from challenge tests. Therefore, it is considered that further studies on vaccination will make it possible to prevent this infection.

Red spot disease is one of the devastating bacterial diseases of cultured eels in Japan. The first outbreaks were observed in 1971 and the causative agent was identified as Pseudomonas anguilliseptica.In this paper, the pathological changes of infected fish, epidemiology, characteristics and pathogenicity of the causative agent, and immunology are briefly reviewed.The disease has three epidemiological characteristics: 1) it prevailed in brackish-water ponds, 2) prevailed when the water temperature was below 20°C or ceased when it rose to 27°C, and 3) prevailed among the Japanese eels (Anguilla japonica). These characteristics were found to be closely related to the nature of the causative agent.Based on these findings, the following methods are proposed as prophylaxis: 1) For the culture of Japanese eels, water temperature of 27°C or above and freshwater ponds are preferable, and 2) European eels are recommended for the ponds where the above conditions are not provided.

Therapeutic experiments were made on red spot disease by using artificiallyinfected eels (Anguilla japonica). As a result, oxolinic acid, nalidixic acid and piromidic acid administered by bathing or orally were found to be effective to control the disease.

The first outbreak of bacterial kidney disease of salmonid (BKD) in HOKKAIDO, Japan occurred in the autumn of 1973. Subsequently, nine outbreaks of BKD were confirmed in Hokkaido and recently occurrence of this disease in the mainland of Japan was also documented by the author.Currently, this disease is being monitored with keen interest in Japan.In this review, the pathology of the infected fish, causative organisms and therapeutic or preventing measures of BKD based on previous reports are discussed. The advantage of two diagnostic techniques (a precipitation reaction and a coagglutination test by specific antibodies absorbed protein A-containing staphylococci with heat extracted antigen of BKD) are presented.

Occurrence of BKD was confirmed at 5 fish farms, 3 hatcheries and 2 aquaria for eight species of salmonid (Oncorhynchus keta, O. gorbuscha, O. kisutch, O. tschawytscha, O. nerka, O. masou, Salmo gairdneri irideus and S.gardneri gairdneri)in Hokkaido during the period from 1973 to March, 1978.Signs of BKD and gram positive bacteria were found among rearing O. masou and O. kisu ch at 2 out of the 5 fish farms, but could not find BKD among anadromous O. keta captured in five rivers in Hokkaido in 1976 and 1977.

Prior to this presentation on preventive measures against fish communicable diseases being theme of this symposium or against fish viral diseases in this subject, the author presents some comments on fish-virus ecology in fish community as the first chapter, subsequently he transfers to the second involving the preventive measures.In the first chapter the author points out that two transmission of vertical and horizontal infection involve in fish viral contagions and the sexual products coming from latently IPN-infected male brood make the vertical transmission spread easily by artificial fertilization with the products which never contain neutralizing antibody to IPNV.In the second he reviews on the virucidal effects of iodine phore, hard water or salt water and a derivative of an ascorbic acid(unpublished)and on the effect of ultraviolet irradiation as well as prophyractic effects to IPN, IHN, VHS by means of artificial active immunity, and on interferon efficacy. He emphasizes that the combined measures with active immunity and an interferon are worthy to be put in an idea as one of the prophyractic measures against viral epizootics occurring at an early stage in the developement of fish like the host of IPN, IHN and others.He stresses that the preventive measures employed can be polytechnic and must impose two significances of controlling the virus in the environmental water for fishes and of inactivating the agent invaded in the animals. The former can be fulfilled with environmental hygienic technology involved using disinfectants or virucidals as the chemical treatment and ultraviolet irradiation as the physical. The latter can be or may be achieved with artificially enhanced fish physiological activity like artificial immunity and artificial interferon induction as well as these combined measures.

An economical, efficacious vaccine delivery system for immunizing fish has been developed which employs a liquid spray apparatus operated at pressures up to 7.0 kg/cm2 (0 to 100 lb/in2). A bacterin consisting of formalin-killed Vibrio anguillarum culture was both antigenic and immunogenic when sprayed on coho salmon(Oncorhynchus kisutch)and rainbow trout(Salmo gairdneri). The technique, referred to as spray(shower)vaccination, was found to confer higher levels of immunity against virulent V. anguillarum than oral vaccination.